Jimmy,
GOOD point on the possibility of a cracked tube in the tank! Copper doesn't seem to do that, but most manufactured tanks use brass tubing. Brass DOES crack along its length sometimes, due to vibration and possibly also to the ingredients in the fuel. (Ever seen the green slime that forms in old tank tubes that haven't held fuel in a while? That's a chemical reaction...)
So, if a tube is split, Joe, replace it with copper! If you have to go into the tank to check, might as well replace them all with copper.
'Nother thought on your 25SS: if you have a muffler, have you run it muffler pressurized? Other than small chunks of grit floating around between tank and needle valve, I dunno.
Oh, and does it restart easily right after it has shut off in flight?